Tandem thin-film silicon solar cell and method for manufacturing the same

ABSTRACT

A tandem thin-film silicon solar cell comprises a transparent substrate, a first unit cell positioned on the transparent substrate, the first unit cell comprising a p-type window layer, an i-type absorber layer and an n-type layer, an intermediate reflection layer positioned on the first unit cell, the intermediate reflection layer including a hydrogenated n-type microcrystalline silicon oxide of which the oxygen concentration is profiled to be gradually increased and a second unit cell positioned on the intermediate reflection layer, the second unit cell comprising a p-type window layer, an i-type absorber layer and an n-type layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 12/412,581, filed Mar. 27, 2009, which in turn claims priority toKorean Patent Application No 10-2008-0049000, filed May 27, 2008, theentireties of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present embodiment relates to a tandem thin-film silicon solar celland a method for manufacturing the same.

Recently, energy has become the most serious problem that impacts onhuman's future life due to the high oil price and the global warmingcaused by an excessive emission of CO2. Many technologies for renewableenergy have been developed, such as wind power generation, bio-energy,hydrogen fuel cells and the like. However, solar energy is almostinfinite and clean energy as the origin of all energies, and hence solarcells using sunlight have become into the spotlight.

The power of sunlight incident on the surface of the earth is 120,000TW. Theoretically, if only 0.16% of the surface area of land on theearth is covered with solar cells having a conversion efficiency of 10%,the solar cell can generate power of 20 TW per year, which is two timesgreater than global energy consumption.

Practically, the global solar cell market was explosively developed withall annual growth rate of 40% during the past 10 years. Currently, 90%of the solar cell market is shared by bulk silicon solar cells such assingle-crystalline silicon solar cells or multi-crystalline siliconsolar cells. However, production of solar-grade silicon wafer which is amain source does not catch up with explosive demand, and the solar-gradesilicon wafer is running short all over the world. Therefore, thesolar-grade silicon wafer is a crucial factor in lowering productioncost.

On the other hand, thin-film silicon solar cells using a absorber basedon hydrogenated amorphous silicon (a-Si:H) can reduce the thickness ofsilicon to less than 1/100 of that of silicon used in the bulk siliconsolar cells. Further, it is possible to produce large-size and low-costthin-film silicon solar cells.

However, the so-called Staebler-Wronski effect is blocking thecommercialization of the thin-film silicon solar cells. TheStaebler-Wronski effect refers to degradation caused by thephotocreation of dangling bond accompanied by non-radiativerecombination of electron-hole pairs photogenerated in an absorber basedon amorphous silicon when light is irradiated onto a thin-film siliconsolar cell.

A large number of studies were conducted during the past thirty years toreduce light-induced degradation of an amorphous silicon-based intrinsicabosorber. As a result, it has been found that kinds of two edgematerials at a phase boundary between amorphous silicon andmicrocrystalline silicon are good intrinsic absorbers with a lowlight-induced degradation ratio. One is hydrogenated intrinsicprotocrystalline silicon (i-pc-Si:H) which exists just before theamorphous silicon to microcrystalline silicon transition. The other ishydrogenated intrinsic microcrystalline silicon (i-pc-Si:H) having acrystal volume fraction of 30 to 50%.

Meanwhile, although the degradation against light soaking is reduced,there is a limit to performance achieved by single-junction thin-filmsilicon solar cells. For this reason, to obtain high stabilizedefficiency, there have been developed a double-junction thin-filmsilicon solar cell in which a top cell based on amorphous silicon isstacked on a bottom cell based on microcrystalline silicon and atriple-junction thin-film silicon solar cell more developed from thedouble-junction thin-film silicon solar cell.

The double-junction or triple-junction thin-film silicon solar cell isreferred to as a tandem solar cell. The open-circuit voltage of thetandem solar cell is the sum of voltages of respective unit cells, andthe short-circuit current of the tandem solar cell is the minimumcurrent among short-circuit currents of the respective unit cells.

In a tandem solar cell which comprises a top cell and a bottom cellforming heterojunction, the optical band gap of an intrinsic absorber isgradually decreased as light is incident from the top cell to the bottomcell. Accordingly, light with a wide range of spectrum is absorbed intothe respective unit cell by separating light, so that quantum efficiencycan be enhanced.

Further, the thickness of an intrinsic absorber of the top cell based onamorphous silicon having relatively serious degradation caused by lightsoaking can be decreased, so that a degradation ratio can be reduced,and accordingly, a highly stabilized efficiency can be obtained.

The stability of the tandem solar cell against light soaking isinfluenced by the stability of an intrinsic absorber of each of the unitcells, against light soaking and the thickness of an intrinsic absorberof the top cell sensitive to light soaking.

Therefore, an intermediate reflection layer capable of strengtheninginternal reflection is interposed between a top cell and a bottom cellin a double-junction thin-film silicon solar cell or between a top celland a middle cell in a triple-junction thin-film silicon solar cell, sothat the thickness of a absorber based on hydrogenated intrinsicamorphous silicon is decreased in the top cell sensitive to lightsoaking, and desired short-circuit current is maintained and improved.Accordingly, a degradation ratio can be reduced, and a highly stabilizedefficiency can be obtained.

At this time, the intermediate reflection layer of the tandem solar cellrequires a transparent material which has a small light absorption, ahigh electric conductivity and a large difference of refractive indicesbetween the intermediate reflection layer and a silicon thin-film.

Since the refractive index of the silicon thin-film is 3.5 to 4.0, amaterial having a refractive index smaller than that of the siliconthin-film has been developed as the intermediate reflection layer. Thematerial is a zinc oxide (ZnO) thin-film having a refractive index ofaround 1.9 (S. Y. Myong et al., Applied Physics Letters, 2007, Vol. 90,p. 3026-3028, Y. Akano et al., EP 1650814A1, Y. Akano et al.,EP1650813A1) or a hydrogenated n-type mixed-phase silicon oxide(n-SiOx:H) thin-film having a refractive index of around 2.0 (C. Das etal., Applied Physics Letters, 2008, Vol. 92, p. 053509, P. Buchlmann etal., Applied Physics Letters, 2007, Vol. 91, p. 143505).

Here, the mixed phase refers to a structure in which crystalline silicongrains are incorporated into the tissue of a hydrogenated amorphoussilicon oxide (a-SiOx:H), and is frequently referred to asnanocrystalline or microcrystalline.

On the Ramam spectrum of a thin-film, a transverse optic (TO) modecrystalline silicon peak exists near 520 nm.

The zinc oxide intermediate reflection layer has excellent transmittanceand vertical electric conductivity, so that the efficiency of the tandemsolar cell can be improved. However, in mass production of large-areasolar cells, problems such as shunts are generated in zinc oxide whenthe zinc oxide intermediate reflection layer is patterned through laserscribing.

Since the hydrogenated n-type mixed-phase silicon oxide thin-film is akind of silicon alloy, laser patterning can be simultaneously performedwith respect to top and bottom cells using the same laser wavelength.Accordingly, the mass production yield of the solar cell can beincreased, and the layout of mass production lines can be simplified. Asthe content of oxygen is increased, the refractive index is decreased,and thus the internal reflection is increased. However, the crystalvolume fraction is decreased, and the electric conductivity is lowered.Therefore, the series resistance is increased, and the fill factor (FF)is decreased.

SUMMARY OF THE INVENTION

In one aspect, a tandem thin-film silicon solar cell comprises atransparent substrate, a first unit cell positioned on the transparentsubstrate, the first unit cell comprising a p-type window layer, ani-type absorber layer and an n-type layer, an intermediate reflectionlayer positioned on the first unit cell, the intermediate reflectionlayer including a hydrogenated n-type microcrystalline silicon oxide ofwhich the oxygen concentration is profiled to be gradually increased anda second unit cell positioned on the intermediate reflection layer, thesecond unit cell comprising a p-type window layer, an i-type absorberlayer and an n-type layer.

The intermediate reflection layer may have a thickness of 10 to 100 nm.

The intermediate reflection layer may have a refractive index of around2.0.

The intermediate reflection layer may have a resistivity of 10² to 10⁵Ωcm.

The n-type layer of the first unit cell may include hydrogenated n-typemicrocrystalline silicon.

The first unit cell may include hydrogenated amorphous silicon.

The second unit cell may include hydrogenated amorphous silicon orhydrogenated microcrystalline silicon.

The solar cell may comprise two or three unit cells.

In one aspect, a method for manufacturing a tandem thin-film siliconsolar cell comprises coating a transparent front electrode layer on atransparent substrate, removing a portion of the transparent frontelectrode layer to form a separation trench using a first patterningprocess, thereby forming a plurality of transparent front electrodes,forming a first unit cell layer on the plurality of transparent frontelectrodes and in the separation trench, the first unit cell layercomprising a p-type window layer, an i-type absorber layer and an n-typelayer, forming an intermediate reflection layer on the first unit celllayer or forming an intermediate reflection layer by oxidizing the firstunit cell layer, the intermediated reflective layer including ahydrogenated n-type microcrystalline silicon oxide of which an oxygenconcentration is profiled to be gradually increased, forming a secondunit cell layer on the intermediate reflection layer, removing portionsof the first and second unit cell layers to form a separation trenchusing a second patterning process, stacking a metal rear electrode layeron the second unit cell layer and in the separation trench formed usingthe second patterning process and removing a portion of the metal rearelectrode layer to forming a separation trench using a third patterningprocess.

When the intermediate reflection layer is formed by oxidizing the firstunit cell layer, an oxygen source gas for forming the intermediatereflection layer may include carbon dioxide or oxygen.

The intermediate reflection layer may have a refractive index of around2.0.

The intermediate reflection layer may have a thickness of 10 to 100 nm.

The intermediate reflection layer may have a specific resistivity of 10²to 10⁵ Ωcm.

Two or three unit cell layers may be formed.

When the intermediate reflection layer is formed on the first unit celllayer, the n-type layer of the first unit cell layer may have athickness of 30 to 50 nm and includes hydrogenated n-typemicrocrystalline silicon.

When the intermediate reflection layer is formed by oxidizing the n-typelayer of the first unit cell layer, the n-type layer of the first unitcell layer may have a thickness of 40 to 150 nm and includeshydrogenated n-type microcrystalline silicon.

When the intermediate reflection layer is formed on the first unit celllayer, the deposition temperature and deposition pressure for stackingthe n-type of the first unit cell layer may be maintained.

The n-type layer of the first unit cell layer and the intermediatereflection layer may be formed in the same reaction chamber.

When the intermediate reflection layer is formed on the first unit celllayer, the pressure fraction of the oxygen source gas for forming theintermediate reflection layer may be increased and then maintainedconstant, or the flow of the oxygen source gas is increased throughmultiple steps.

When the intermediate reflection layer is formed by oxidizing the firstunit cell layer, plasma for forming the n-type layer of the first unitcell layer is turned off, the oxygen source layer for forming theintermediate reflection layer may be introduced, and the plasma may bethen turned on.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating the structure of a tandemthin-film silicon solar cell according to an embodiment.

FIGS. 2 a and 2 b are flowcharts illustrating a method for manufacturinga tandem thin-film silicon solar cell according to an embodiment.

FIGS. 3 a and 3 b are flowcharts illustrating a method for manufacturinga tandem thin-film silicon solar cell according to another embodiment.

DETAILED DESCRIPTION

Reference will now be made in detail embodiments of the inventionexamples of which are illustrated in the accompanying drawings.

Tandem thin-film silicon solar cell has a double-junction structure or atriple-junction structure. However, a tandem thin-film silicon solarcell having a double-junction structure will be described as an examplein FIG. 1.

As illustrated in FIG. 1, a tandem thin-film silicon solar cellaccording to an embodiment comprises a transparent insulating substrate10, a transparent front electrode 20, a first unit cell 30, anintermediate reflection layer 40, a second unit cell 50 and a metal rearelectrode 70.

The transparent front electrode 20 is formed on the transparentinsulating substrate 10 and includes a transparent conducting oxide(TCO).

The first unit cell 30 are positioned on the transparent insulatingsubstrate 10 and the transparent front electrode 20 and comprises ap-type window layer 30 p, an i-type absorber layer 30 i and an n-typelayer 30 n, which are formed using a plasma enhanced chemical vapordeposition (CVD) technique.

The intermediate reflection layer 40 is positioned on the first unitcell 30 and includes a hydrogenated n-type microcrystalline siliconoxide. At this time, an oxyzon concentration of the intermediatereflection layer 40 is profiled to be gradually increased.

The second unit cell 50 is positioned on the intermediate reflectionlayer 40 and comprises a p-type window layer, an i-type absorber layer50 i and an n-type layer 50 n, which are formed using a PECVD technique.The second unit cell 50 may comprise hydrogenated amorphous silicon orhydrogenated microcrystalline silicon.

The metal rear electrode 70 is stacked on the second unit cell 50.

As illustrated in FIG. 1, the tandem thin-film silicon solar cellaccording to the embodiment may further comprise a back reflection layer60 grown using a CVD technique so as to maximize the light trappingeffect between the n-type layer 50 n of the second unit cell 50 and themetal rear electrode 70.

In the tandem thin-film silicon solar cell, the transparent insulatingsubstrate 10 is a portion to which light is primarily incident and hasexcellent transmittance of light. The transparent insulating substrate10 may include a transparent insulative material to prevent an internalshort circuit in the thin-film silicon solar cell.

The transparent front electrode 20 allows incident light to be scatteredin various directions and has durability for hydrogen plasma used toform a microcrystalline silicon thin-film. Accordingly, the transparentfront electrode 20 may include zinc oxide (ZnO).

In the embodiment, the first unit cell 30 comprising the p-type windowlayer 30 p, the i-type absorber layer and the n-type layer 30 n isformed by a radio frequency plasma enhanced chemical vapor deposition(RF PECVD) technique using a frequency of 13.56 MHz or a very highfrequency (VHF) PECVD technique using a higher frequency than 13.56 MHz.With the increase in the plasma excitation frequency, the depositionspeed is increased, and the quality of layers is improved. At this time,the first unit cell 30 may include hydrogenated amorphous silicon.

The intermediate reflection layer 40 includes a oxygen-profiledhydrogenated n-type microcrystalline silicon oxide or a hydrogenatedn-type microcrystalline silicon oxide formed using a subsequentoxidizing process. The intermediate reflection layer 40 is formed on thehydrogenated n-type microcrystalline silicon (n-[mu]c-Si:H) thin-film 30n.

The second unit cell 50 may be formed on the intermediate reflectionlayer 40 using an RF or VHF PECVD technique. The second unit cell 50comprises a p-type window layer 50 p, an i-type absorber layer 50 i andan n-type layer 50 n. At this time, the second unit cell 50 is based onmicrocrystalline silicon or amorphous silicon.

The back reflection layer 60 is deposited on the n-type layer 50 n ofthe second unit cell 50 using a CVD technique and includes zinc oxide(ZnO) so as to maximize the light trapping effect between the n-typelayer 50 n of the second unit cell 50 and the metal rear electrode 70.

The metal rear electrode 70 serves as a rear electrode which reflectslight transmitted into the second unit cell 50. The metal rear electrode70 may include zinc oxide (ZnO) or silver (Ag). The metal rear electrode70 may be grown using a CVD or sputtering technique.

For serial connection in the mass production of thin-film silicon solarcells, a patterning process is performed using a pattern forming methodsuch as laser scribing. Such a patterning process is performed on thetransparent front electrode 20, the second unit cell 50 and the metalrear electrode 70.

Hereinafter, a method for forming an intermediate reflection layer willbe described in detail with reference to FIGS. 2 a, 2 b, 3 a and 3 b.FIGS. 2 a and 2 b are flowcharts illustrating a method for manufacturinga tandem thin-film silicon solar cell according to an embodiment.

As illustrated in FIG. 2 a, a transparent front electrode layer iscoated on a transparent insulating substrate 10 (S210).

A portion of the transparent front electrode layer coated on thetransparent insulating substrate 10 is removed to form a separationtrench using a first patterning process, thereby forming a plurality oftransparent front electrodes 20 spaced apart from each other (S220).

A first unit cell layer is formed in the separation trench formedbetween the plurality of transparent front electrodes 20 and is formedon the transparent front electrode 20 (S230). Here, the first unit celllayer comprises a p-type window layer 30 p, an i-type absorber layer 30i and an n-type layer 30 n.

An intermediate reflection layer 40 including a hydrogenated n-typemicrocrystalline silicon oxide (n-[mu]c-SiOx:H) is formed on the firstunit cell layer (S240). By controlling the flow of carbon dioxideintroduced into a chamber, an oxygen concentration of the intermediatereflection layer 40 is profiled to be gradually increased.

A second unit cell layer is formed on the intermediate reflection layer40 (S250). Here, the second unit cell layer comprises a p-type windowlayer 50 p, an i-type absorber layer 50 i and an n-type layer 50 n.

Portions of the first and second unit cell layers are removed to form aseparation trench using a second patterning process, thereby firmingfirst and second unit cells 30 and 50 (S260).

A metal rear electrode layer is stacked on the second unit cell 50 andis stacked in the separation trench formed using the second patterningprocess (S280). Accordingly, the metal rear electrode layer is connectedto the transparent front electrode 20.

A portion of the metal rear electrode layer is removed to form aseparation trench using a third patterning process, thereby forming aplurality of metal rear electrodes 70 (S290).

To maximize the light trapping effect after the second patterningprocess, a back reflection layer 60 may be formed on the n-type layer 50n of the second unit cell layer using a CVD technique (S270).

To improve an initial efficiency, a buffer layer may be interposedbetween the p-type window layer 30 p and the i-type absorber layer 30 iin the forming of the first unit cell layer (S230). The initialefficiency refers to the efficiency of the solar cell before lightsoaking according to the embodiment.

In the forming of the first unit cell layer (S230), the first unit celllayer comprising p-i-n-type thin-film silicon is formed using an RF orVHF PECVD technique. The n-type layer 30 n of the first unit cell layerhas a thickness of 30 to 50 nm and comprises a hydrogenated n-typemicrocrystalline silicon (n-μc-Si:H) thin-film. When the n-type layer 30n of the first unit cell layer has a thickness of 30 nm or thicker, then-type layer 30 n has high electrical conductivity. When the n-typelayer 30 n of the first unit cell layer has a thickness of 50 nm orthinner, excessive light absorption due to the increase of thickness isprevented.

At this time, a source gas for forming the hydrogenated n-typemicrocrystalline silicon (n-μc-Si:H) thin-film may include silane(SiH₄), hydrogen (H₂) and phosphine (PH₃).

After the n-type layer 30 n comprising the hydrogenated n-typemicrocrystalline silicon (n-μc-Si:H) thin-film is formed, an oxygensource gas such as carbon dioxide (CO2) is introduced into a reactionchamber (S241 a) while maintaining a flow rate of the source gas,deposition temperature and deposition pressure as illustrated in FIG.2B. Since the flow rate of the source gas, the deposition temperatureand the deposition pressure are maintained and a mixed gas containingthe source gas and the oxygen source gas is introduced into the reactionchamber, the n-type layer 30 n of the first unit cell layer and theintermediate reflection layer 40 may be formed in the same reactionchamber.

At this time, the flow rate of the oxygen source gas is controlled by amass flow controller (MFC) (S242 a). That is, when the mixed gas in thereaction chamber is set to have a predetermined flow rate, the MFC maycontrol the pressure fraction of the oxygen source gas in the mixed gasto be increased and then maintained constant. Accordingly, theoxygen-profiled intermediate reflection layer 40 is formed on the firstunit cell layer (S243 a).

The intermediate reflection layer 40 includes a hydrogenated n-typemicrocrystalline silicon oxide (n-μc-SiOx:H), and an oxygenconcentration of the hydrogenated n-type microcrystalline silicon oxide(n-μc-SiOx:H) is profiled to be gradually increased. For example, when adistance between a first position of the inside of the hydrogenatedn-type microcrystalline silicon oxide (n-μc-SiOx:H) and the transparentinsulating substrate 10 is greater than a distance between a secondposition of the inside of the hydrogenated n-type microcrystallinesilicon oxide (n-μc-SiOx:H) and the transparent insulating substrate 10,an oxygen concentration at the first position is greater than an oxygenconcentration at the second position.

The MFC allows the flow rate of the oxygen source gas to be increasedthrough multi-steps (S242 a), thereby forming the intermediatereflection layer 40 including a hydrogenated n-type microcrystallinesilicon oxide (n-μc-SiOx:H) (S243 a). Accordingly, the oxygenconcentration of the hydrogenated n-type microcrystalline silicon oxide(n-μc-SiOx:H) is profiled to be gradually increased.

The thickness of the intermediate reflection layer 40 may be 10 to 100nm. When the thickness of the intermediate reflection layer 40 is 10 nmor thicker, the internal reflection of light can be sufficientlyperformed. When the thickness of the intermediate reflection layer 40 is100 nm or thinner, light can be sufficiently supplied from the firstunit cell 30 to the second unit cell 50, and it is possible to preventlight absorption caused by the intermediate reflection layer 40 and toprevent unnecessary increase in series resistance between the first andsecond unit cells 30 and 50.

In the embodiment, the intermediate reflection layer 40 may have aresistivity of 10² to 10⁵ Ωcm and a refractive index of around 2.0.Accordingly, the intermediate reflection layer 40 has a high verticalelectrical conductivity.

Therefore, since the rapid lowering of the crystal volume fraction isprevented, the degradation of a vertical electrical conductivity isprevented. A refractive index or optical band gap is not discontinuouslychanged but is continuously changed at the boundary between the n-typelayer 30 n of the first unit cell layer and the intermediate reflectionlayer 40.

Accordingly, it is possible to prevent the detect density at theheterogeneous boundary between the n-type layer 30 n of the first unitcell layer and the intermediate reflection layer 40 from being rapidlyincreased, and light absorption caused by the intermediate reflectionlayer 40 can be minimized.

FIGS. 3 a and 3 b are flowcharts illustrating a method for manufacturinga tandem thin-film silicon solar cell according to another embodiment.

As illustrated in FIG. 3 a, a transparent front electrode layer iscoated on a transparent insulating substrate 10 (S310).

A portion of the transparent front layer is removed to form a Separationtrench using a first patterning process, thereby forming a plurality oftransparent front electrodes 20 (S320).

A first unit cell layer is formed on the transparent front electrode 20,and is formed in the separation trench formed by the first patterningprocess (S330). Here, the first unit cell layer comprises a p-typewindow layer 30 p, an i-type absorber layer 30 i and an n-type layer 30n.

Oxygen atoms are decomposed from an oxygen source gas such as oxygen orcarbon dioxide by plasma, and the oxygen atoms are diffused intomicrocrystalline silicon (n-[mu]c-Si:H) of the n-type layer 30 n,thereby forming an intermediate reflection layer 40 including ahydrogenated n-type microcrystalline silicon oxide (n-μc-SiOx:H) (S340).That is, in another embodiment, the intermediate reflection layer 40 isformed by oxidizing the n-type layer 30 n of the first unit cell layer.

A second unit cell layer is formed on the intermediate reflection layer40 (S350). Here, the second unit cell layer comprises a p-type windowlayer 50 p, an i-type absorber layer 50 i and an n-type layer 50 n.

Portions of the first unit cell layer, the intermediate reflection layer40 and the second unit cell layers are removed to form a separationtrench using a second patterning process, thereby forming first andsecond unit cells 30 and 50 (S360).

A metal rear electrode layer is stacked on the second unit cell 50 andin the separation trench formed using the second patterning process(S380). Accordingly, the metal rear electrode layer is connected to thetransparent front electrode 20.

A portion of the metal rear electrode layer is removed to forming aseparation trench using a third patterning process, thereby forming aplurality of metal rear electrodes 70 (S390).

To maximize the light trapping effect after the second patterningprocess, a back reflection layer 60 may be formed on the n-type layer 50n of the second unit cell layer using a CVD technique (S370).

To improve an initial efficiency, a buffer layer may be interposedbetween the p-type window layer 30 p and the i-type absorber layer 30 iduring the formation of the first unit cell layer (S330).

When the first unit cell layer is formed using an RF or VHF PECVDtechnique (S330), the n-type layer 30 n of the first unit cell layer mayhave a thickness of 40 to 150 nm and comprises a hydrogenated n-typemicrocrystalline silicon (n-μc-Si:H) thin-film. At this time, a sourcegas for the formation of the n-type layer 30 n of the first unit celllayer may include silane (SiH₄), hydrogen (H₂) and phosphine (PH₃).

Meanwhile, as illustrated in FIG. 3 h, an end of the deposition of thehydrogenated n-type microcrystalline silicon (n-μc-Si:H) thin-film isperformed by the turn-off of plasma (S341 b), i.e., the end of plasmaproduction. After the deposition of the hydrogenated n-typemicrocrystalline silicon (n-μc-Si:H) thin-film is ended, the threesource gases are exhausted from the reaction chamber while maintainingthe deposition temperature (S342 b).

After the source gases are exhausted, the post-process base pressure inthe reaction chamber may be 10⁻⁵ to 10⁻⁷ Torr. After the pressure in thereaction chamber reaches the post-process base pressure, an oxygensource gas such as oxygen (O₂) or carbon dioxide (CO2) is introducedinto the reaction chamber (S343 b). The post-process pressure in thereaction chamber is maintained constant by a pressure controller and anangle valve, which are connected to the reaction chamber.

At this time, a flow rate of the oxygen source gas introduced into thereaction chamber may be (0 to 500 sccm, and the pressure in the reactionchamber may be 0.5 Torr. If the flow rate of the oxygen source gas is 10sccm or more, the oxygen diffusion speed is increased. If the flow rateof the oxygen source gas is 500 sccm or less, the unnecessary increaseof gas cost is prevented. When the pressure in the reaction chamber is0.5 Torr, an appropriate oxygen diffusion speed is secured, and theincrease of gas cost is prevented.

Thereafter, the oxygen source gas is decomposed by turning on plasma(S344 b), i.e., by generating plasma, and oxygen atoms are produced. Ifthe surface of the hydrogenated n-type microcrystalline silicon(n-μc-Si:H) thin-film of the n-type layer 30 n is oxidized by the oxygenatoms (S345 b), an intermediate reflection layer 40 comprising ahydrogenated n-type microcrystalline silicon oxide (n-μc-SiOx:H)thin-film is formed on the hydrogenated n-type microcrystalline silicon(n μc-Si:H) thin-film of the n-type layer 30 n (S346 b).

After the oxidizing process is ended by controlling a time of theoxidizing process, the thickness of the hydrogenated n-typemicrocrystalline silicon (n-μc-Si:H) thin-film is decreased to 30 to 50nm. As the thickness of the hydrogenated n-type microcrystalline silicon(n-μc-Si:H) thin-film is increased, the crystal volume fraction isincreased and the vertical electrical conductivity is also increased. Inanother embodiment, the hydrogenated n-type microcrystalline silicon(n-μc-Si:H) thin-film having high crystal volume fraction is easilypost-oxidized by penetrating oxygen into a grain boundary thereof.

The surface of the hydrogenated n-type microcrystalline silicon(n-μc-Si:H) thin-film is converted into the intermediate reflectionlayer 40 comprising hydrogenated n-type microcrystalline silicon oxide(n-μc-SiOx:H) thin-film through the post-oxidation process. Accordingly,the rapid lowering of the vertical electrical conductivity is prevented,and the refractive index is decreased.

As illustrated in FIGS. 2 a to 3 b, in the methods of manufacturing atandem thin-film silicon solar cell according to the embodiments, thefirst to third patterning processes for serial connection are performedusing techniques such as laser scribing.

Accordingly, to form a large-area solar cell, the respective unit cellsand the intermediate reflection layer are simultaneously patterned usinglaser with the same wavelength, so that the production yield of thetandem thin-film silicon solar cell is increased, and the layout of massproduction lines can be simplified.

In the present embodiments, the p-type window layers 30 p and 50 p arelayers doped with an impurity such as a group III element, and thei-type absorber layers 30 i and 50 i are intrinsic silicon layers. Then-type layers 30 n and 50 n are layers doped with an impurity such as agroup V element.

Meanwhile, in the tandem thin-Film silicon solar cell and the method Formanufacturing the same according to the present embodiments, ap-i-n-p-i-n-type double-junction thin-film silicon solar cell having aplurality of and a p-i-n-p-i-n-p-i-n-type triple junction thin-filmsilicon solar cell will be described.

As aforementioned, in the p-i-n-p-i-n-type double-junction thin-filmsilicon solar cell, an intermediate reflection layer 40 including ahydrogenated n-type microcrystalline silicon oxide (n-μc-SiOx:H) with arefractive index of around 2.0 is formed between first and second unitcells.

In the double-junction thin-film silicon solar cell, an i-type absorberlayer of the first unit cell may include any one of hydrogenatedintrinsic amorphous silicon (i-a-Si:H), hydrogenated intrinsicprotocrystalline silicon (i-pc-Si:H), hydrogenated intrinsicprotocrystalline silicon multilayer (i-pc-Si:H multilayer), hydrogenatedintrinsic amorphous silicon carbide (i-a-SiC:H), hydrogenated intrinsicprotocrystalline silicon carbide (i-pc-SiC:H), hydrogenated intrinsicprotocrystalline silicon carbide multilayer (i-pc-SiC:H multilayer),hydrogenated intrinsic amorphous silicon oxide (i-a-SiO:H), hydrogenatedintrinsic protocrystalline silicon oxide (i-pc-SiO:H) or hydrogenatedintrinsic protocrystalline silicon oxide multilayer multilayer).

In the double-junction thin-film silicon solar cell, an i-type absorberlayer of the second unit cell may include any one of hydrogenatedintrinsic amorphous silicon (i-a-Si:H), hydrogenated intrinsic amorphoussilicon germanium (i-a-SiGe:H), hydrogenated intrinsic protocrystallinesilicon germanium (i-pc-SiGe:H), hydrogenated intrinsic nanocrystallinesilicon (i-nc-Si:H), hydrogenated intrinsic microcrystalline silicon(i-pc-Si:H) or hydrogenated intrinsic microcrystalline silicon germanium(i-pc-SiGe:H).

Meanwhile, in the p-i-n-p-i-n-p-i-n-type triple-junction thin-filmsilicon solar cell, a unit cell (hereinafter a middle cell) positionedin the middle of three unit cells may correspond to the first or secondunit cell explained in the present embodiments.

When the middle cell is the first unit cell, the second unit cell ispositioned in contact with or adjacent to a metal rear electrode of thetriple-junction thin-film silicon solar cell. When the middle cell isthe second unit cell, the first unit cell is positioned in contact withor adjacent to a transparent front electrode of the triple-junctionthin-film silicon solar cell.

Further, when the middle cell is the second unit cell, an intermediatereflection layer including a hydrogenated n-type microcrystallinesilicon oxide (n-μc-SiOx:H) is formed between the first and second unitcells.

Furthermore, an intermediate reflection layers including a hydrogenatedn-type microcrystalline silicon oxide (n-μc-SiOx:H) with a refractiveindex of around 2.0 may be formed between every two adjacent unit cellsof the three unit cells.

In the triple-junction thin-film silicon solar cell, an i-type absorberlayer of the unit cell in contact with or adjacent to the transparentfront electrode may include any one of hydrogenated intrinsic amorphoussilicon (i-a-Si:H), hydrogenated intrinsic protocrystalline silicon(i-pc-Si:H), hydrogenated intrinsic protocrystalline multilayer(I-pc-Si:H multilayer), hydrogenated intrinsic amorphous silicon carbide(i-a-SiC:H), hydrogenated intrinsic protocrystalline silicon carbide(i-pc-SiC:H), hydrogenated intrinsic protocrystalline silicon carbidemultilayer (i-pc-SiC:H multilayer), hydrogenated intrinsic amorphoussilicon oxide (i-a-SiO:H), hydrogenated intrinsic protocrystallinesilicon oxide (i-pc-SiO:H) or hydrogenated intrinsic protocrystallinesilicon oxide multilayer (i-pc SiO:H multilayer).

In the triple-junction thin-film silicon solar cell, an i-type absorberlayer of the middle may include any one of hydrogenated intrinsicamorphous silicon germanium (i-a-SiGe:H), hydrogenated intrinsicprotocrystalline silicon germanium (i-pc-SiGe:H), hydrogenated intrinsicnanocrystalline silicon (i-nc-Si:H), hydrogenated intrinsicmicrocrystalline silicon (i-pc-Si:H) or hydrogenated intrinsicmicrocrystalline silicon germanium carbon (i-[mu]c-SiGeC:H). Anintrinsic absorber of the bottom cell may include any one ofhydrogenated intrinsic amorphous silicon germanium (i-a-SiGe:H),hydrogenated intrinsic protocrystalline silicon germanium (i-pc-SiGe:H),hydrogenated intrinsic nanocrystalline silicon (i-nc-Si:H), hydrogenatedintrinsic microcrystalline silicon (i-μc-Si:H) or hydrogenated intrinsicmicrocrystalline silicon germanium (i-μc-SiGe:H).

The foregoing embodiments and advantages are merely exemplary and arenot to be construed as limiting the present invention. The presentteaching can be readily applied to other types of apparatuses. Thedescription of the foregoing embodiments is intended to be illustrative,and not to limit the scope of the claims. Many alternatives,modifications, and variations will be apparent to those skilled in theart.

What is claimed is:
 1. A method for manufacturing a tandem thin-filmsilicon solar cell, comprising: coating a first electrode layer on asubstrate; removing a portion of the first electrode layer to form afirst separation trench using a first patterning process, therebyforming a plurality of first electrodes; forming a first unit cell layeron the plurality of first electrodes and in the first separation trench,the first unit cell layer comprising a p-type window layer, an i-typeabsorber layer and an n-type layer; forming an intermediate reflectionlayer comprising a hydrogenated n-type microcrystalline silicon oxide ofwhich an oxygen concentration is profiled to be gradually increased fromthe light incident side; forming a second unit cell layer on theintermediate reflection layer; removing portions of the first and secondunit cell layers to form a second separation trench using a secondpatterning process; stacking a second electrode layer on the second unitcell layer and in the second separation trench formed using the secondpatterning process; and removing a portion of the second electrode layerto form a third separation trench using a third patterning process. 2.The method according to claim 1, wherein the intermediate reflectionlayer is formed by oxidizing the n-type layer of the first unit celllayer, and an oxygen source gas for forming the intermediate reflectionlayer comprises carbon dioxide or oxygen.
 3. The method according toclaim 1, wherein the intermediate reflection layer has a refractiveindex of around 2.0.
 4. The method according to claim 1, wherein theintermediate reflection layer has a thickness of 10 to 100 nm.
 5. Themethod according to claim 1, wherein the intermediate reflection layerhas a specific resistivity of 10² to 10⁵ Ωcm.
 6. The method according toclaim 1, wherein two or three unit cell layers are formed.
 7. The methodaccording to claim 1, wherein the intermediate reflection layer isformed on the first unit cell layer, and the n-type layer of the firstunit cell layer has a thickness of 30 to 50 nm and compriseshydrogenated n-type microcrystalline silicon.
 8. The method according toclaim 1, wherein the intermediate reflection layer is formed byoxidizing the n-type layer of the first unit cell layer, and the n-typelayer of the first unit cell layer has a thickness of 40 to 150 nm andcomprises hydrogenated n-type microcrystalline silicon.
 9. The methodaccording to claim 1, wherein the intermediate reflection layer isformed on the n-type layer of the first unit cell layer, and thedeposition temperature and deposition pressure for stacking the n-typelayer of the first unit cell layer are maintained.
 10. The methodaccording to claim 9, wherein the n-type layer of the first unit celllayer and the intermediate reflection layer are formed in the samereaction chamber.
 11. The method according to claim 1, wherein theintermediate reflection layer is formed on the first unit cell layer,and the pressure fraction of the oxygen source gas for forming theintermediate reflection layer is increased and then maintained constant,or the flow of the oxygen source gas is increased through multiplesteps.
 12. The method according to claim 1, wherein the intermediatereflection layer is formed by oxidizing the n-type layer of the firstunit cell layer, and plasma for forming the n-type layer of the firstunit cell layer is turned off, the oxygen source layer for forming theintermediate reflection layer is introduced, and the plasma is thenturned on.
 13. A method for manufacturing a tandem thin-film siliconsolar cell, comprising: coating a first electrode layer on a substrate;removing a portion of the first electrode layer to form a firstseparation trench using a first patterning process, thereby forming aplurality of first electrodes; forming a first unit cell layer on theplurality of first electrodes and in the first separation trench;forming an intermediate reflection layer comprising a hydrogenatedn-type microcrystalline silicon oxide of which an oxygen concentrationis profiled to be gradually increased from the light incident side;forming a second unit cell layer on the intermediate reflection layer;removing portions of the first and second unit cell layers to form asecond separation trench using a second patterning process; stacking asecond electrode layer on the second unit cell layer and in the secondseparation trench formed using the second patterning process; andremoving a portion of the second electrode layer to form a thirdseparation trench using a third patterning process.
 14. The methodaccording to claim 13, wherein the first unit cell layer comprises ap-type window layer, an i-type absorber layer and an n-type layer.